U.S. Attorney General Eric Holder says President Obama does not have the authority to use an armed drone against a non-combatant American on U.S. soil.

Holder cited that conclusion in a letter to Sen. Rand Paul, R-Ky., who engaged in a 13-hour filibuster to challenge Obama's drone program.

Paul declared victory after hearing of Holder's letter.

"For 13 hours yesterday, we asked him that question, so there is a result and a victory," the Kentucky senator told Fox News. "Under duress and under public humiliation, the White House will respond and do the right thing."

In his letter to Paul, Holder wrote:

"It has come to my attention that you have now asked an additional question: Does the president have the authority to use a weaponized drone to kill an American not engaged in combat on American soil? The answer to that question is no."

Both Holder and Carney have said that, as commander in chief, Obama reserves the right to do what is necessary to protect the country.

Said Carney: "In an event like an attack like Pearl Harbor or an attack like 9/11, obviously the president has the constitutional authority to take action to prevent those kinds of attacks."

Paul had sought to hold up the nomination of John Brennan to be CIA director; Brennan, a White House counterterrorism adviser, has been involved in directing the drone program.